Multimode-interference optical multiplexers are used as optical multiplexers in optical integrated circuits. Loss reduction and reflection reduction in the optical multiplexers are required along with the proceeding of integration of a plurality of light sources.
For instance, Japanese Patent Application Laid-Open No. 2010-237376 (Patent Document 1) discloses an optical modulator that includes a Mach-Zehnder waveguide disposed on a dielectric substrate. An example of the dielectric is lithium niobate. The Mach-Zehnder waveguide includes an output Y-multiplexer having a multimode waveguide through which light that has been multiplexed passes. A subsidiary output waveguide is connected to a location where the multimode waveguide is changed into a main output waveguide. The subsidiary output waveguide is a waveguide for higher-order mode where radiation-mode light is output.